1.) Mozart's sudden and unexpected death
2.) That at the time no one knew what actually caused his death
3.) Mozart's belief that someone was poisoning him
4.) The mysterious appearance of an anonymous "man in grey" commissioning Mozart for a "Mass for the Dead" (The Requiem)
5.) Salieri confessing to murdering Mozart
You might think the Fifth Point would be enough to convict Salieri, except for one problem. Salieri's "confession" came many years later when he was confined in an asylum, he was completely insane by then. The movie retained that element, if you notice. Salieri, after his attempted suicide, was strangely not taken to a hospital but an insane asylum. Don't you find that curious? You might wonder, however, "What if Salieri really did kill Mozart? Is it possible?" It might be possible, but the chances are incredibly slim. Salieri was merely a business colleague, not a close friend. In order to poison or kill Mozart successfully he would have to be on very close terms which Salieri, in reality, never established. At best he could only intrigue from afar at court, but never slip Mozart poison or accomplish the equivalent of Mozart's famous death scene in "Amadeus".
